Extraocular muscle sideslip and orbital geometry in monkeys.

The belly of each extraocular muscle is elastically coupled to both the globe and orbit. The dependence of muscle planes on gaze angle must be determined experimentally. In monkeys, radio-opaque markers were implanted along the upper and lower margins of a lateral rectus. A scleral search coil was implanted in the other eye. With the eye in various gaze positions, X-ray images were made to show the LR in the lateral view. We found that as the eye rotates vertically over 50 deg (+/- 25 deg), the point of tangency of the LR with the globe slips an average of 5.1 mm vertically with respect to the globe, allowing this point--and so the muscle plane--to remain approximately fixed relative to the orbit. The results of quantitative orbital dissections are presented in support of the sideslip calculations.